How to use List type with Codable? (RealmSwift)

Question

Problem is List type does not conform to Codable, the below class cannot be insert to Realm.

for example,

class Book: Codable {
    var name: String = ""
    var author: String = ""
    var tags = [String]()
}

Consider the above class conforms to Codable, if store this class to Realm, it needs to use List<Object> type instead of [String]

class Book: Object, Codable {
    @objc dynamic var name: String = ""
    @objc dynamic var author: String = ""
    var tags = List<Tag>()

    required convenience init(from decoder: Decoder) throws {
        self.init()
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        author = try container.decode(String.self, forKey: .author)
        tags = try container.decode(List<Tag>.self, forKey: .tags)   // this is problem.
    }
}

class Tag: Object, Codable {
    @objc dynamic var string: String = ""

    required convenience init(from decoder: Decoder) throws {
        self.init()
        let container = try decoder.container(keyedBy: CodingKeys.self)
        string = try container.decode(String.self, forKey: .string)
    }
}

To conform to Codable, it should be implement Decodable protocol. (required convenience init(from decoder: Decoder) throws)

But, List type does not conform to Codable(Decodable), it is impossible to use Codable if the class has List type.

How to resolve this issue?

Thanks,

Answer 1

We can use an extension to make List conform to Codable:

extension List : Decodable where Element : Decodable {
    public convenience init(from decoder: Decoder) throws {
        self.init()
        var container = try decoder.unkeyedContainer()
        while !container.isAtEnd {
            let element = try container.decode(Element.self)
            self.append(element)
        }
    } }

extension List : Encodable where Element : Encodable {
    public func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        for element in self {
            try element.encode(to: container.superEncoder())
        }
    } }

I also got an extension for RealmOptional, if others need.

https://gist.github.com/ansonyao/41137bb3cbbca8ef31a13b6bc96ee422

Answer 2

You are almost there. Inside the initializer, you can initialize the list using the decoded array. Basically, change

tags = try container.decode(List<Tag>.self, forKey: .tags)   // this is problem.

to

let tagsArray = try container.decode([Tag].self, forKey: .tags)   
tags = List(tagsArray) // Now you are good

As pointed out in the comments the List constructor no longer works like this

You now want:

tags.append(objectsIn: tagsArray)